To provide an opener in which an opening work of an envelope can be safely performed without a hand touching a blade by a state in which the blade is located in a body, and that can be used as a one piece cutting cutter of a newspaper, a magazine or the like at a time.

When an opening work of an envelope is performed, a cutter blade (4) disposed to synchronize with an adjustment dial (3) is pinched at a space with a packing (5) to be fixed in the inside of a body by tightening a fastening screw (1). Under the condition, it is possible to open by passing an end of the envelope from a gap that exists in a position of a cutting edge. Moreover, when being used as a one piece cutting cutter, the fastening screw (1) is loosened, and the adjustment dial (3) is rotated, thereby it becomes possible to cut one paper because being regulated from a state in which the cutter blade (4) is stored in the body to a constant exposure state by no stage.